Question

2. How many valence electrons does the element located in group 1, period 4 contain?
A. 1 B. 3 C. 7 D. 8
3. How many electron shells does Argon (atomic number 18) contain?
A. 7 B. 5 C. 3 D. 1
4. Which of the following elements is not metal?
a. Magnesium (12) b. Tungsten (74) c. Lithium (3) d. Krypton (36)
5. Draw an atomic model of Sodium (11) below.
Level 2: Understanding chemical equations
6. Photosynthesis is a process in which plants take in water and carbon dioxide to create
glucose and oxygen. Below is the equation. Label the following parts of an equation:
subscript, coefficient, product, reactant

7. True or false: The coefficient within a chemical equation represents the number of groups of
molecules or compounds.
Explain.
8. Chemical equations can be shown by creating models. Below is a model of a chemical
equation. Below is a model of a chemical equation. What does coefficient 2 in front of the
HCl represent?
Explain.
9. A chemical equation can be balanced or unbalanced. Looking at the equation below shows
evidence that proves that this equation is balanced or unbalanced.
Is the equation above balanced? ______________ Explain:
10. What is the difference between an open and closed system? Explain.

Answer 1

2. Answer: A (1)
- Elements in group 1 (the alkali metals) have 1 valence electron. The element in group 1, period 4 is potassium (K).

3. Answer: C (3)
- Argon (Z = 18) is in period 3, so it has 3 electron shells (electron configuration: 2, 8, 8).

4. Answer: d. Krypton (36)
- Mg, W, and Li are metals; krypton is a noble gas (nonmetal).

5. Atomic model of Sodium (Z = 11)
- Electrons: 11 → shell arrangement 2, 8, 1.
- Nucleus (most common isotope Na-23): 11 protons, 12 neutrons.
Simple diagram (text):
[Nucleus: 11 p, 12 n]
Shell 1: ●● (2 electrons)
Shell 2: ●●●●●●●● (8 electrons)
Shell 3: ● (1 electron)

Level 2

6. Photosynthesis equation:
6CO2 + 6H2O → C6H12O6 + 6O2
- Subscript: the small numbers inside formulas (e.g., the 2 in CO2, the 12 in C6H12O6) that show how many atoms of an element are in one molecule.
- Coefficient: the large number in front of a formula (e.g., the 6 in front of CO2) showing how many molecules (or moles) of that species are involved.
- Reactants: left side (6CO2 and 6H2O).
- Products: right side (C6H12O6 and 6O2).

7. True.
- The coefficient represents the number of molecules (or moles) of that substance. Example: 2H2O means two water molecules. Coefficients are used to balance the number of atoms on each side of an equation.

8. The coefficient 2 in front of HCl means there are two molecules (or two formula units) of HCl.
- So 2 HCl contains 2 hydrogen atoms and 2 chlorine atoms in total. Coefficients multiply the entire molecule (not just one atom).

9. I don’t see the specific equation you’re referring to. Please paste it.
- How to check: count the number of atoms of each element on the reactant side (using coefficients × subscripts) and compare to the product side. If every element has the same total on both sides, the equation is balanced.
- Example: Photosynthesis above is balanced because C:6 = 6, H:12 = 12, O:18 = 18 on both sides.

10. Difference between an open and closed system (chemistry context)
- Open system: matter and energy can be exchanged with the surroundings (example: an open beaker — vapor can escape).
- Closed system: energy can be exchanged but matter cannot (example: a sealed, rigid container where gas cannot enter or leave).
- (Also note: an isolated system exchanges neither matter nor energy, but that’s less common in lab setups.)
- Practical consequence: in a closed system total mass inside stays constant (useful when tracking conservation of mass in reactions).